Field of Invention
This invention relates to conducting polymers and their preparation, and more particularly relates to a method for forming a conjugated heteroaromatic polymer, and to a conjugated heteroaromatic homopolymer or copolymer formed by the method.
Description of Related Art
During the past few decades, backbone-conjugated conducting polymers, such as polyacetylenes, polyanilines, polyaromatics, polyheteroaromatics, poly(aromatic vinylene)s and poly(heteroaromatic vinylene)s, have raised great research interests in both industrial and academic communities, because of their great application potentials and their novel electronic, optical, electrooptical, and opto-electronic properties. Conducting polymers have been demonstrated to have great potentials for many important applications, such as antistatic, ESD, EMI-shielding, cable-shielding, radar-shielding, high frequency capacitor, rechargeable battery, anti-corrosion, gas separation membranes, smart window, chemical sensor, bio-sensor, solar cell, light-emitting diode, electrochromic display, field effect transistor, organic memory device, lithography, via-hole electroplating, and nonlinear optical materials.
Among the conjugated conducting polymers, polyheteroaromatics and particularly polythiophenes have attracted great attentions recently due to their easier processability and better thermal stability. Regarding the conventional methods, most of the polyheteroaromatics have been synthesized from heteroaromatics via oxidative polymerization either electrochemically or chemically. For example, U.S. Pat. No. 4,697,001 discloses synthesis of polypyrrole from pyrrole via oxidative chemical polymerization using metal-containing oxidant, such as FeCl3 or Fe(OTs)3.
Polythiophenes have been in general prepared either from 2,5-unsubstituted thiophenes or from 2,5-dihalogenated thiophenes. For example, polythiophene can be prepared from thiophene using metal-containing oxidants such as FeCl3, MoCl5, and RuCl3 (Jpn. J. Appl. Phys. 1984, 23, L899), or from 2,5-dibromothiophene via metal-catalyzed polycondensation polymerization using the combined reagent of Mg metal and Ni(0) catalyst (U.S. Pat. No. 4,521,589). Recently, the metal-catalyzed polycondensation method has been modified by many research groups, such as Reike's and McCullough's, for making regioregular poly(3-substituted thiophenes) from 3-substituted 2,5-dibromo-thiophenes using various combination of metal-containing reagents: such as Li/naphthalene/ZnCl2/Ni(II) or Pd(0) complex (U.S. Pat. No. 5,756,653), organomagnesium reagent/Ni(II)-complex (U.S. Pat. No. 6,166,172), organomagnesium reagent/ZnCl2/Ni(II)-complex (U.S. Pat. No. 7,572,880), and organomagnesium reagent/MnCl2/Ni(II) complex (US 2010/0234478A1).
Up to now, there is only one reported prior method of preparing polythiophene from 2-bromothiophenes (U.S. Pat. No. 6,602,974). For example, McCullough indicated that the above regioregular poly(3-substituted thiophenes) can also be made from 3-substituted 2-bromo-thiophene via a three-step reaction by treating the monomer in the first step with a strong base LDA (lithium diisopropylamine, prepared freshly from the reaction of diisopropylamine with n-butyl lithium) under cryogenic temperatures at −40° C. for 40 minutes, followed by the addition of MgBr2 at −60° C. for about 1 hour in the second step, and then by the addition of Ni(II)-complex at −5° C. in the third step, and finally allowed the reaction to proceed at room temperature for additional 18 hours.
All the above mentioned prior methods have encountered the disadvantage of being contaminated with substantial amount of metal impurities that can cause detrimental effects to the optimal performance, long term stability, and the lifetime of their application articles and devices. Furthermore, most of the conventional methods involve use of either a strong base (such as organolithium reagent, organomagnesium reagent, and LDA) or a reactive metal (such as activated Zn metal, Mg metal, and Li metal), or in some cases the use of both reagents. These reagents are reactive toward the monomers that contain a proton group having a pKa value of less than about 40 (such as S—H, O—H, N—H, acetylenic proton, α-hydrogen to a carbonyl group or to other electron-withdrawing group, and all the C—H groups except those of alkyl, alkoxy, phenyl, and vinyl) and also reactive toward electrophilic functional groups (such as carbonyl, carbonate, nitrile, imino, nitro, nitroso, sulfoxide, sulfonyl, and sulfonyl, phosphonyl, phosphinyl, epoxy, alkyl halide, and other similar groups).
Such reactivity will cause great limitations on the allowable functional groups appearing on the thiophene monomers. Similar limitations would also occur to the conventional chemical oxidative polymerization method, wherein the employed strong oxidant causes the undesirable oxidation side reactions to some of the functional groups. The high reactivity of both the above strong base and active metal will also greatly limit the applicable reaction and/or processing solvent media. These strong bases and reactive metals are also in general moisture- and air-sensitive, which requires the use of expensive and complicated production apparatus, facility, handling and manufacturing process. These reactive reagents also cast great industrial potential hazards. In addition, the above methods often involve the use of either the cryogenic temperatures (such as −40° C. to −78° C.) or the reflux temperatures for long hours, which would further increase the production costs and energy consumptions. Further, these transition metal complexes are not only very expensive but also environmentally concerned.
Though a specific polythiophene derivative, poly(3,4-ethylenedioxythiophene) (PEDOT), has been prepared via a catalyst-free solid-state oxidative polymerization by heating the solid crystals of 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) as a monomer at some elevated temperatures below its melting temperature (96-97° C.), as described in J. Am. Chem. Soc. 2003, 125, 15151-15162, the same polymerization did not proceed in its melt or solution state. Such solid-state polymerization method is also only applicable to limited cases due to its unique requirements for the steric arrangement between two neighboring dihalogenated monomers within the crystal. In the same report, they had also found that the addition of protonic acid catalyst (such as HBr) did not lead to any changes to the dibromo-monomer (see the footnote 24 thereof).
On the other hand, U.S. Pat. No. 6,891,016 discloses that in presence of protonic acid or Lewis acid, non-brominated 3,4-ethylenedioxythiophene (EDOT) changes significantly to yield an equilibrium reaction mixture that contains unreacted monomer (˜50%) and non-conjugated dimeric and trimeric thiophenes (˜50%), instead of polymers. Though U.S. Pat. No. 7,951,901 discloses that the mixture of EDOT and DBEDOT can somehow undergo polymerization in presence of protonic acid or Lewis acid, the polymerization requires heating at high temperatures (80-90° C.) for long time (5-11 h) and only gives PEDOT in a poor yield (40-60%) with a rather low conductivity of 10−2-10−7 S/cm. Furthermore, this method is only applicable to 3,4-dialkoxy-substituted thiophenes.
Thus, there is a great need for an effective, energy saving, and environmentally friendly method for making polythiophenes and polyheteroaromatics in general.